Adding a consonant not present in a Qur'anic word, or omitting one that is present, disturbs its form. One cause is inserting an unintended sound around a doubled or unvowelled consonant.
Addition: introducing a sound that is not part of the prescribed word.
Omission: failing to pronounce a consonant that belongs to the word.
Correction: pronounce the word slowly according to its letters and marks, then practise it in connected recitation.
Both the written form and a reliable oral model matter; a merely similar sound should not be assumed correct.